1. Field of the Invention
The present invention is directed to a block for use with a bridge crane wherein the block is adjustable in the field to adjust the lateral spacing of the sheaves in order to optimize the system geometry for the particular application and to minimize wear on the components.
2. Prior Art
Bridge cranes and accompanying blocks operate on the principle of mechanical advantage which is obtained through the use of multiple sheaves or pulleys. The greater the number of sheaves, the greater the load that can be lifted for a given line pull. This allows for crane designs that use relatively smaller winches to lift relatively heavier loads. The sheaves are attached to the load via a load block and a fitting.
Bridge cranes are often used in applications where the bridge crane will ride on a beam or beams and a block is suspended from wire line extending from the bridge crane.
The load is raised or lowered as a rotating drum on the bridge crane pulls in or lets out wire rope. As the rope moves, the block is either raised or lowered away from the drum assembly. The change in the geometry due to the raising and lowering of the load causes the so-called “fleet angle” to change. This fleet angle can become excessive which will cause increased wire rope wear and increased wear on the sheave flange. The wear in the system is of concern as it may ultimately decrease the strength of the rope and reduce the desired and necessary design factor of the rope.
Standard bridge crane systems often specify particular block geometry in order to minimize the amount of rope and sheave wear along the full range of block travel. This results in the maximum rope and sheave wear at the lowest and highest positions of the block. Special applications may result in the block operating primarily at or near the highest or the lowest position. For example, in lifting tall pieces, the block may operate primarily at its highest position. Conversely, in lifting shorter pieces, the block may operate primarily at its lowest position. While it would be possible to replace the entire block including sheaves, depending on the particular application, this is time consuming and requires an inventory of different blocks.
Accordingly, for a given set of lifting height parameters, the optimum sheave spacing may be identified.
Also in the past, other types of block arrangements have been utilized. For example, Swanson (U.S. Pat. No. 5,603,420) discloses floating sheaves in FIGS. 11 through 14 which are movable along the shafts. Oftentimes, however, the floating sheaves are not suitable for harsh environments associated with bridge crane activity.
Morris et al. (U.S. Pat. No. 6,408,956) discloses movable sheaves. The movable sheaves slide to reduce the fleet angle in the system and reduce the amount of moment applied to the piston and drill string.
Larralde (U.S. Pat. No. 3,936,034) discloses a block with multiple sheaves having a mechanism to minimize the fleet angle of the line by varying the angle of the crown block sheaves. The adjustment to the spacing of the wire line is not obtained by moving the sheaves but by moving the ropes away from a rotational center to unused sheaves in order to try to balance the moments generated in the system.
Additionally in the past, various designs have provided to ease reeving of lines on the sheaves. Wilkinson (U.S. Pat. No. 4,098,492) discloses sheaves having removable guards 24 and 25 to quickly reeve the line.
Accordingly, it would be desirable to provide a replacement bridge crane block wherein the spacing between the sheaves is adjustable in the field.
It would also be desirable to provide a field adjustable replacement bridge crane block having a center pin which is long enough to accommodate various chosen spacing widths between the sheaves.
It would also be desirable to provide a field adjustable replacement bridge crane block having a replaceable center pin for selected spacing of the sheaves.
It would also be desirable to provide a field adjustable replacement bridge crane block having a nut locking mechanism to lock a nut onto the center pin.
It would also be desirable to provide a field adjustable replacement bridge crane block having a sheave housing plate which is locked against rotation.
It would also be advantageous to provide a field adjustable replacement bridge crane block having a removable outer housing plate for access to the sheave.